The statist Left has long supported a planned economy run by a centralized state. As we're all aware, the project has failed miserably and produced massive tyranny everywhere it's ever been tried, without any exceptions.
The anarchist Left supports a planned economy as well, but with a few tweaks. First, they want it to be more decentralized. (How this is going to work with the "confederation of Communes" or whatever they want is anyone's guess.) They also picture it being "voluntary" and more democratic, with everyone having an opportunity to participate. This, they seem to believe, will iron out the wrinkles in the central-planning slacks.
But this is still basically impossible. Why?
Every single day of our lives, we make numerous, spontaneous economic decisions. In a given day we might choose to fill up with gas, buy a Coke at a vending machine, replace a light bulb, whatever. As anyone with experience in marketing research knows, most people cannot come close to accurately predicting the economic decisions they will make (how much they'll spend and on what), even in the very near future.
Think of all the different things you might buy in just a single trip to the grocery store: eggs, hamburger meat, tooth paste, shoe laces, shampoo, deodorant, aspirin, head phones, whatever. Think of all the different, often far-away places each of these items came from, as well as how they were made and distributed. Then think about all the diferent, far-away places the producers had to buy the materials from, just to produce those items (and so on).
Despite the different origins and producers of each item, they are all just sitting there, immediately available for you to consume at this location, at a second's notice. It is hard to realize just how convenient and efficient all of this is, until it's gone.
When you check out of the grocery store, you don't have to argue with every other stranger shopping there over who deserves to get what, or why. You don't have to defend your choices. You don't have to worry about how many groceries other people are buying, or whether they're freeloading. You rarely have to worry about what you need not being available, because supply and demand are taken into account. Again, the ease and convenience of this is taken for granted by most on the left.
And of course, you don't have to directly haggle with the people who produced each individual good you're buying.
Under a planned economy, even a "voluntary" one, all of this goes out the window, and we're left with an unworkable nightmare.
The complicated, rapidly changing and widespread nature of an entire economy simply cannot be planned, especially not through "voting" or "workers' councils." (Sure, there have been small communes, but most of them have been failures and all of them have had to leech off of the capitalist system for survival.) To think otherwise is utopian and downright religious...certainly not scientific in any sense of the word. Capitalistic businesses exist on every corner of practically every country--we know that they work. Anarcho-leftie societies do not exist anywhere, because they can't possibly sustain any large, free, heterogeneous group of people.
What market anarchists understand is that wishing for a certain society doesn't make it possible. While I don't like quoting Ayn Rand, she was right when she said, "facts are facts, independent of man's feelings, wishes, hopes or fears." Socialism and planned economies will always fail miserably, regardless of how many brainy intellectuals, do-gooders, or crackpot theorists wish otherwise. This is a matter of simple science.
The next time you're at the grocery store, be thankful you're not under a planned economy.